AITAH for telling my sister who her husband voted for which is now seriously making her consider divorce?

My sister’s husband and I have been best friends since childhood so I was very happy when my sister married him, because at his core, he’s a good man. We have been friends for a really long time because we’re both very sporty and like to go on runs, hikes, play sports etc. We rarely have any deep talks, we usually just joke around and have casual convos.

For this election, he voted for Trump which surprised me, and I asked if he had told my sister about it because my sister has a strong stance on it. He said he lied to her, and that it wasn’t a big deal and to let it go. Now normally, I would have let it go, because who he voted for doesn’t really bother me, but since this was about my sister, I told him he either had to tell her or I would. He begged me not to tell, but I had no choice, because ultimately, my loyalty lies with my sister. I told my sister about it, and she is now seriously considering divorce. This has also ruined my friendship with him even though I did’t want it to. However, my sister is very thankful I told her, and she is also very sorry and apologetic that it has ruined my friendship with my best friend.

Was I the AH?